The present invention generally relates to a floating piston for a pressure storage device and more particularly to a packing arrangement for sealing the sliding surface between the piston and a cylinder wall of the pressure storage device.
Piston pressure storage devices of this type are used to store pressure energy which is released as needed to a hydraulic system, such as, to supply a short-term requirement for a high fluid volume to alternate impacts and vibrations, to maintain pressure in the case of a pump failure, to compensate for leaking losses or for pressure- and volume-variations in a closed system, and the like.
Since no large pressure gradients between the sliding surface of the floating piston and the cylinder wall of the storage device appear at the packing, the danger of leaking or of the passing of the pressurized media to the other side of the piston appear to be slight. However, in pressure storage devices, such as hydropneumatic energy storage devices, the reliable separation of the hydraulic pressure liquid from the volume of pressurized gas is extremely important, because the penetration of a liquid in the gaseous space would interfere with proper functioning and would reduce the useful volume of the gaseous space. Also, any gas entering the liquid space, possibly after dissolution in the liquid, may be carried into the hydraulic system and can cause problems at other locations of this system, such as, favor the appearance of cavitation. In the past, a separating wall in the shape of an elastic bladder has been used. Regrettably, such devices have the disadvantage of a short operational life.
Prior art piston packings are known utilizing a pressure conduit situated between the frontal surface of the piston on the pressure side and the lateral wall of a circumferential piston ring groove containing a packing material. The pressure acting on the lateral sealing surface is propagated to all sides of the packing material and forces the external circumferential surface of the packing material against the sliding surface of the cylinder. This arrangement provides a seal capable of withstanding higher medium pressure and prevents the passage of the medium across the seal. However, this configuration provides no assurance that the pressure introduced by the pressure channel will actually be converted into a radially outward pressure force, that is, uniform over the entire axial dimension of the packing material.
A somewhat more advantageous configuration is known, wherein the pressure channel opens not at the frontal surface of the packing but into its internal circumference so that the pressure introduced in this manner is directed radially outward against the packing material. Such a design may be found, for example, in U.S. Pat. No. 2,799,523. The disadvantage of this configuration is that the pressurized medium exerted against the rearside of the packing material may readily leak along the surface of the packing groove and to the other side, so that a reliable separation between the cylinder chambers divided by the piston is not assured. Accordingly, this is not an acceptable arrangement for floating pistons where medium separation is necessary.
Increasing the reliable separation of the pressurized media can be achieved by means of two successively arranged packings on the piston although difficulties can arise because an uncontrolled pressure can build up in the annular gap between the packings which can drive the packing material apart resulting in increased friction and finally destruction of the packing material.
Alternatively, difficulties may arise when the pressure in the annular gap between the packings is too low. This is common in piston pressure storage devices wherein the piston moves in a relatively thin cylinder liner, surrounded by a pressure vessel in communication with the gas of the cylinder liner adapted to absorb the operating pressure. When the piston pressure storage device with the cylinder liner is not under pressure and then subjected to a sudden increase in pressure, the pressure in the annular gap between the packings will not build up immediately. Concurrently, the cylinder liner is exposed to a high pressure from the outside, so that it may be deformed and bent inwardly over the axial distance between the two packings, whereby the surface of the piston may jam or in any case be exposed to increased friction.